The present invention relates generally to time delay estimation and more particularly to circuits for time delay estimation in laser scanning.
Increasing complex systems require increasingly accurate and fine-grained time delay estimation. For example, some laser scanning systems measure a time delay between a sent and received laser pulse to determine the distance to an object. The time delay between the sending of the pulse and the receiving of the reflected pulse is very small and must be measured with great accuracy—on the order of less than 10 nanoseconds—in order to properly determine the desired distance measurement. Conventional methods of time delay estimation are either prohibitively expensive or unable to accurately detect such small time intervals.
In a laser scanning system, the distance to a remote object is measured by reflecting some energy of a short laser pulse off the object. When the pulse is emitted, some of its energy is diverted immediately and is sent to an avalanche photo diode. The difference in time between the time the pulse is emitted and the time the reflected pulse is received at the emitter, multiplied by the speed of light, provides an estimate of the distance to the remote object. In order for the distance measurement to have accuracy on the order of about a millimeter, the time estimate must be accurate to within a few picoseconds.
Conventional techniques of time delay estimation in laser scanning are described in U.S. Pat. No. 6,665,055, entitled “Light-Wave Rangefinder Using a Pulse Method” (Ohishi), U.S. Pat. No. 5,619,317, entitled “Light-Wave Distance Meter Based on Light Pulses” (Oishi), and U.S. Patent Application No. 2005/0052952, entitled “Time Interval Measurement Device” (Panek).
Ohishi and Oishi disclose techniques for introducing an electrical pulse into a tuned filter. This has the effect of stretching the pulse into a series of damped oscillations. To further reduce the analog measurement bandwidth, this waveform is periodically sampled at a low frequency with a small increase of time delay between each sample. However, as discussed above, these methods fail to provide sufficient accuracy for short time interval estimation and thus cannot provide a quality distance measurement.
Panek utilizes improvements in the speed and cost of high speed samplers to use a slightly different approach. Panek discloses sampling in real time when the bandwidth of the tuned filter is narrower than half the sampling bandwidth. However, only a small part of the pulse energy is used in such an approach. Only a small dynamic range of pulse durations can be measured because, as longer pulses are introduced, the filter response is necessarily diminished. This approach also compromises system accuracy by separately introducing a calibration pulse into each channel.
Related methods of time delay estimation used a Nutt interpolator. However, the Nutt interpolator cannot measure pulse widths wider than the resolution of the filter used and information is lost. Accordingly, such a method cannot properly account for the increased resolution accuracy required in modern time delay estimation.
Accordingly, improved systems and methods of time delay estimation are required.